Isolation and structure elucidation of larvicidal compounds from Laggera alata and Clausena anisata

Abstract

Laggera alata is in the Asteraceae family, and. Clausena anisata is in the family of Rutaceae both plants have shown bioactivity against several diseases. Malaria is by far the most important insect transmitted disease. There is no so far vaccine to prevent infection caused by A. gambiae mosquito and the malaria parasite is continually developing resistance to the available drugs, so vector control is the best option. Dried, ground and weighed 600g of aerial parts for each plant (L. alata and C. anisata) were sequentially extracted with hexane, ethyl acetate, chloroform, acetone and methanol. The solvents were removed by rotor evaporation under vacuum to give five extracts for each species. Fresh whole plants of L. alata and C. anisata were subjected to hydro-distillation in a modified Clevenger-type apparatus for at least four hours according to the British pharmacopoeia. The essential oils obtained was 7g of C. anisata oils constituting 4.25% and 5g of L. alata oils accounting for 2.78% in a yield of w/w after drying over anhydrous sodium sulphate respectively. The oils were subjected to GC, GC-MS to determine the composition. In L. alata oils, the major compounds were: 2, 5-dimethoxy-para -Cymene 24.4%, cis-Chrysanthenol 11.8%. The oils from C. anisata gave the following major compound composition: β-Phelandrene (Limonene) 20.1%, Germacrene-D 18.8%, γ-Terpinene 13.8%. The bioassays were performed with third instar larvae of A. gambiae s.s, carried out in triplicate using 20 larvae for each replicate assay. From larvicidal assay the LC50 of the L. alata oils was found to be 273.38 mg/l and that of C. anisata was 75.96 mg/l. The LC99 for L. alata was 507.75 mg/l and C. anisata was 256.80 mg/l. LC50 of L. alata hexane fraction was 1161.30 mg/l and the corresponding LC99 was 2734.91 mg/l. The C. anisata ethyl acetate fraction gave LC50 as 2095.46 mg/l and LC99 was 4438.75 mg/l. The ethyl acetate fraction of C. anisata was subjected to GC-MS analysis to determine total chemical composition. The hexane fraction of L. alata was subjected to chromatographic separation leading to isolation and purification of compound 1 and 2. Spectroscopic analysis was done to elucidate the structure of new eudesmane sesquiterpenoids: 3β-angeloyloxy-4β-hydroxy-eudesm-7, 11-en-8-one (1) and 3β-angeloyloxy- 4β-acetoxy-11-hydroxy eudesm-6-en-8-one (2). Application of these extracts to larval habitats may lead to promising results in malaria and mosquito management programmes. The isolated larvicidal compounds can be used as lead compounds for environmentally friendly and biodegradable larvicides.